1. Field of the Invention
This invention relates generally to the field of gas conduit tubes, and more particularly concerns the construction of leak proof lap joints between telescopically communicating exhaust pipes.
2. Description of the Prior Art
Most automotive exhaust systems provide a conduit for exhaust gas to pass from the point of combustion to a remote outlet, passing through a series of treatment devices such as a muffler, catalytic converter, and resonator. The exhaust gas may also be passed through other devices where it performs a work function, such as in an air cleaner aspirator. In order that such exhaust systems may be assembled, means must be provided for the effective coupling of a number of lengths and shapes of tubing.
The most common way of joining pipes to construct an exhaust conduit is to utilize a lap joint, wherein succeeding tubes of smaller diameter are fitted telescopically into preceding tubes of larger diameter, and clamped in the region where the tubes overlap. Clamping means which encircle the outer tube are tightened down so as to crimp the outer tube onto the inner tube in the region of overlap of the tube ends.
Since most exhaust tubes are made of steel, they generally have one end formed with a plurality of slots running parallel to the longitudinal axis of the tube extending from the tube end inwardly for a distance of one and one half to three inches in order to facilitate the crimping of the outer tube to the inner tube. A number of these slots are placed around the circumference of the outer tube at its end. A typical muffler connecting tube is shown in the patent to Bryant, U.S. Pat. No. 2,825,421. A similar slotted tube end construction is shown in the patent to L. H. Billey, U.S. Pat. No. 3,137,553, for an air cleaner aspirator.
The slots in such an outer tube configuration allow the outer tube to be deformed, or crimped inwardly onto an inner tube placed telescopically within the outer tube, by the action of a clamp encircling the outer tube in the region of the slots. The most common clamping means for use in lap joint construction is the so-called guillotine or saddle-type clamp. Such a clamp comprises a U-bolt having a radius of curvature generally equal to the outside radius of the outer exhaust tube, and a saddle which slides on the free legs of the U-bolt. The saddle and U-bolt are forced towards each other by means of a pair of nuts threadedly engaging the ends of the U-bolt legs. When the nuts are tightened, the saddle and U-bolt cooperate to squeeze the tubes which they encircle.
In the conventional lap joint construction, the inner tube has an outside diameter which is smaller than the inside diameter of the outside tube, and the inner tube extends into the outer tube so as to cover the outer tube slots from the inside. The saddle clamp is then positioned around the outer tube at a point intermediate the length of the slots. When the saddle clamp is tightened, it crimps the outer tube in a localized area. As a result of this localized crimping and the differences in diameters of the inner and outer tubes, gaps are formed between the tube walls upstream of the crimped region. These gaps allow gas to pass between the walls, into the slotted region of the outer tube, and thus escape axially down the slots to the end of the outer tube and radially from the slots around the clamping ring. This leakage of exhaust gas at the lap joint not only presents a safety hazard, because of the escape of noxious exhaust fumes, but also presents a noise problem--the pulsating escape of gases is highly audible.
In order to produce a safe, noise-free exhaust system, the lap joint couplings utilized in the system must be substantially leak-proof. As was pointed out above, the conventional slotting arrangement on the outer tube allows exhaust gas to escape both radially and axially from the slots. While the use of a band-type clamp, which clamps are known in the industry, may prevent to some extent the escape of gas radially from the lap joint, such clamps cannot prevent escape of gas down the slots and out the slot ends. Several devices have been developed to prevent the axial escape of gas at the lap joints. Sipler, U.S. Pat. No. 3,572,774 discloses the use of a flexible gasket which extends past and around the end of the outer tube to engage the inner tube wall. Heimstra et al., U.S. Pat. No. 3,944,265 discloses the use of a band of ductile metal, which is passed around the adjacent ends of the two door members and circumferentially stressed beyond its elastic limit to conform intimitely with the outer surfaces of the members throughout their entire circumferences.
Due to the cost of the special material of which stretchable bands must be made, or of the extra part comprising the gasket seal, these alternative solutions to the problem are not entirely satisfactory for all applications.